When a plurality of signals of subscriber lines are transmitted through the same optical fiber, the plurality of signals of subscriber lines are accommodated with, for example, Time Division Multiplexing (TDM). A signal of a subscriber line accommodated in a TDM transmission line is often referred to as a “traffic demand” or simply as a “demand”, and therefore the subscriber line accommodated in the TDM transmission line is hereinafter referred to as a “demand”.
For the TDM transmission line, the SDH (Synchronous Digital Hierarchy) and the OTN (Optical Transport Network), etc., are put into practical use, for example. Commonly, one TDM transmission line such as an SDH transmission line is set in one optical fiber, before Wavelength Division Multiplexing (WDM) is introduced. Therefore, before the WDM is introduced, an optical fiber is respectively laid between nodes (e.g., telephone exchanges), and a TDM transmission line is set between the nodes.
Assume, for example, that, in a transmission system including nodes A-E as illustrated in FIG. 1A, an optical fiber is respectively laid between A and B, B and C, C and D, and D and E. Further, assume that one TDM transmission line is set in one optical fiber. Namely, TDM transmission lines #1, #2, #3, and #4 are respectively set between A and B, B and C, C and D, and D and E. In this case, when a demand which transmits a signal between the node A and the node E is provided, this demand is accommodated in the TDM transmission lines #1, #2, #3, and #4. Namely, there is one method for accommodating this command (or, one accommodation pattern). Hereinafter, a demand which transmits a signal between a node i and a node j is sometimes referred to as a “demand i-j”.
In recent years, WDM technology has been widely spread, and an Optical Add/Drop Multiplexer (OADM) is often provided in each node. As illustrated in FIG. 2, the OADM can branch an optical signal of a desired wavelength from a received WDM optical signal and guide it to a subscriber. In addition, the OADM can add a subscriber signal into a WDM optical signal. Further, the OADM can transmit an optical signal included in the received WDM signal to the next node without converting it into an electrical signal.
Here, assume that an OADM is provided in each of the nodes B, C, and D in the transmission system illustrated in FIG. 1A. Then, the transmission system can provide TDM transmission lines #5-#10 illustrated in FIG. 1B, in addition to the TDM transmission lines #1-#4 illustrated in FIG. 1A. In this case, demands A-E may be accommodated with a desired pattern from among seven patterns illustrated in FIG. 1C in addition to the accommodation pattern illustrated in FIG. 1A. Namely, the demands A-E may be accommodated in, for example, the TDM transmission lines #5, #3, and #4 or the TDM transmission lines #1, #2, and #6. As described above, by using the WDM technology, the flexibility in a design of TDM transmission lines which will accommodate a demand increases.
In, for example, Japanese Laid-Open Patent Application Publication No. 5-290023 and Japanese Laid-Open Patent Application Publication No. 2012-73705, a technology relating to a network design is described.
In a transmission system in which a plurality of signals are transmitted with TDM, which demand is accommodated in which TDM transmission line is determined in advance. At this time, each demand is preferably accommodated in a TDM transmission line in order to efficiently utilize a communication resource (e.g., a bandwidth).
However, as described above, in the transmission system using the WDM technology, the number of patterns of a combination of TDM transmission lines which will accommodate a demand is large. For that reason, in a large-scale transmission system, in which there are many nodes, it takes a huge amount of time to determine an efficient accommodation pattern. In addition, when the efficient accommodation pattern is determined using a computer, a huge storage capacity is needed for executing the calculation.